Why is this opening?
This is not normal after any surgical procedure. You should go to the surgeon who performed the procedure and if they are not available you should see a medical provider right away.
It is difficult to answer your question based on limited information that you have supplied. I would recommend you see your operating surgeon ASAP if you have any questions. In general, it is normal to have some pain after surgery, but not normal to have lots of pain with puss. Again, you need to contact your operating surgeon and see him/her to answer your question.